The aims of the proposed project are to relate the structure and function of a steroid-regulated gene cluster in Drosophila melanogaster. The gene cluster is the 68C salivary gland polytene chromosome puff; it contains the DNA encoding three different RNAs that are expressed coordinately, and are messenger RNAs of three polypeptides secreted by the larval salivary glands. The expression of these RNAs is controlled by the insect steroid hormone ecdysterone. The specific aims of the project are three: to study the level and nature of the effects of hormones on expression of the 68C puff RNAs, to use in vivo and in vitro mutagenesis of the puff or its cloned DNA to locate regulatory DNA sequences in the puff, and to compare the DNA sequence organization of the puff in several species of Drosophila other than melanogaster. The proposed work involves the disciplines of molecular genetics, classical genetics and biochemistry; its ultimate goal is to understand developmental, tissue specific, and hormone controlled gene regulation in development. The relation of this goal to human health is that any progress made will enable a more advanced understanding of the aberrations in gene expression that manifest themselves as birth defects and cancer, and will lead to a better comprehension of the normal mechanisms of gene regulation by the endocrine system.